This invention relates to optical switches employing controllable asynchronous or phase-mismatched coupling between two parallel optical guides and having electrical adjustability of both cross-over and straight-through states.
In prior experimental investigation of alternatives for optical switches, it has been proposed that optical directional couplers employing dielectric strip guides or "channel" guides fabricated in electrooptical material or semiconductor material and controlled by an applied electric field can serve as amplitude modulators or as switches of information-carrying optical channels. Examples of such proposals are U.S. Pat. No. 3,408,131 (1968) to Mr. N. S. Kapany and the article by Mr. E. A. J. Marcatili in the Bell System Technical Journal, Volume 48, page 2071 (1969). In principle, the switching function can be obtained by control of either the coupling strength between the two waveguides or of the difference .DELTA..beta. of the two propagation constants, the latter being the phase mismatch or asynchronism of the respective guides.
Nevertheless, it was soon recognized that the control of phase mismatch is preferable as it requires drive voltages that are about two orders of magnitude smaller than the direct control of coupling strength. Implementations of such control techniques that utilize changes in the propagation constants or phase velocities of two adjacent waveguides to perform switching are described in the articles by Mr. M. Papuchon et al, Applied Physics Letters, Volume 27, page 289 (September 1975) and by J. C. Campbell et al., Applied Physics Letters, Volume 27, page 202 (August 1975). Since this type of switching involves changes in the difference between the propagation constants which are usually designated as .beta., the switching is commonly referred to in the art as .DELTA..beta. switching. The first of the latter two articles describes structures in lithium niobate and the latter describes structures in gallium arsenide.
The devices just mentioned appear to perform acceptably as amplitude modulators; but they apparently do not produce optical switching that meets the stringent crosstalk requirements demanded by a typical communication system. One of the reasons for this difficulty is the fact that the just-mentioned directional couplers allow the electrical adjustment of only one of the two switching states.
It is therefore desirable to improve the prior art optical directional couplers to overcome the problems of such limited electrical adjustment.